Optical amplifiers have attracted a great deal of attention for use in fiber optic communications systems. They exhibit high gain, low noise, negligible crosstalk and intermodulation distortion, bit-rate transparency, and polarization insensitive gain. These properties make optical fibers superior to semiconductor devices as amplifiers in fiber optic systems. Moreover, fiber-based amplifiers do not require conversion from photon energy to electrical energy as do semiconductor devices.
In a communications system of any significant size, there is typically a distribution network that includes long communication paths and nodes where the network branches. In such a network, amplifiers are required in order to maintain the amplitude of the signal and the integrity of any data it carries in route between a source and destination. For these amplifiers to function properly, they must exhibit high small signal gains and/or high output saturation powers.
Application of erbium-doped optical fibers as amplifiers has received a lot of attention recently because the characteristic gain bandwidth of these fibers is within the third telecommunications window of 1.5 microns commonly used in fiber optic communications systems. Recent efforts in designing erbium fiber amplifier systems has focused on providing the optimum wavelength for pumping the erbium, with regard to both the efficiency of the small signal gain and availability of long-lived pump sources having relatively high optical output quality. Diode lasers have been a focus of interest as pumping sources because of their high electrical-to-optical conversion efficiencies, long lives and small size. Because of these qualities, laser diodes are typically considered to be the most commercially viable pump sources.
Many wavelengths and pump sources have been tried for directly pumping erbium in erbium-doped fibers. For example, in order of their representative gain efficiency the following pump wavelengths have been explored: 980 nm (4 dB/mW), 1480 nm (2.5 dB/mW), 532 nm (1.5 dB/mW), 660 nm (0.5 dB/mW), and 807 nm (0.2 dB/mW). Directly pumping erbium at the wavelengths of 980 or 1480 nm is currently the most popular approach and both pump wavelengths have been directly provided by diode lasers.
In terms of gain efficiency, directly pumping an erbium-doped fiber at a wavelength of 980 nm is the most attractive, but availability of diode lasers at this wavelength is limited and serious issues concerning the lifetimes of strained-layer InGaAs diodes at this wavelength have not been resolved. As an alternative to pumping at a wavelength of 980 nm, erbium amplifiers have been pumped on the edge of the 1550 band at 1480 nm with InGaAsP diode lasers. The gain efficiency of pumping at 1480 nm is not as high as pumping at 980 nm, but pump diodes at 1480 nm currently have longer demonstrated lifetimes than diodes at 980 nm. There are some additional advantages to pumping at a wavelength of 1480 nm. One advantage is that the pump wavelength is close to the signal wavelength and therefore it is relatively easy to ensure single-mode operation of the erbium fiber at the pump wavelength, a condition necessary to achieve efficient gain. The gain profile for pumping at 1480 nm is also broader than for pumping at 980 nm, an advantage for wavelength division multiplying (WDM) applications. Conversely, the noise performance of a 1480 nm pumped erbium fiber amplifier is intrinsically worse than a 980 nm pumped amplifier due to the lower value of the inversion parameter.
Although some degree of success has been achieved toward the goal of providing an erbium-doped fiber amplifier having high small signal gain efficiency and long life, the power saturation output remains relatively low (R. Baker, Physics World, pp. 41-44, March 1990.). The power saturation output of a fiber is typically given as the output power (dBm) that decreases the small signal gain by three (3) decibels (dB). In general, a fiber is characterized by a steady small signal gain that is maintained over a wide range of power outputs (i.e., photon density). As the input signal of the fiber increases in power, the power of the output signal reaches a saturation condition that causes the small signal gain to decrease with any additional increase in the power of the input signal.
In order to increase the level of power saturation in erbium-doped fiber pumping at the wavelength of 532 nm has attracted a great deal of attention because of the availability of frequency-doubled, diode-pumped Nd:YAG lasers of high output power and high spatial mode quality (A. Righetti et al., Electr. Lett., 26(5), p. 330, 1990). The overall efficiency (i.e., electron-to-photon conversion) of any erbium amplifier system based on frequency-doubled diode-pumped lasers, however, is very low (i.e., 2% at 532 nm compared to 15% for the fundamental frequency) precluding its use in some applications such as repeaters.
Although laser diodes at 808 nm are widely available, their use as pumps for erbium fiber amplifiers has generally been dismissed because of the strong excited-state absorption (ESA) of erbium at 807 nm, which seriously limits the efficiency of such amplifiers. As a result of the intense ESA in the 807 nm pump band for an erbium fiber, only small gains have been observed (e.g., six dB-T.J. Whitely, "Laser Diode Pumped Operation of Er.sup.3+ -Doped Fibre Amplifiers" Electron. Lett. 1988, No. 24; p. 1537)
A laser based on a ytterbium-erbium co-doped phosphatic glass rod has been pumped by a Nd:YAG laser at its 1064 nm output. Hanna et al., "A 1.54 .mu.m Er Glass Laser Pumped By A 1.064 .mu.m Nd: YAG Laser", Optics Communications; Vol. 63, No. 6, Sep. 15, 1987: pp. 417-420. Hanna et al. reports experimental results indicating the co-doped phosphatic glass rod may make a laser of reasonable gain and efficiency for use as a source of 1540 nm radiation in a communications system.
Hanna et al. speculates that a single mode, co-doped ytterbium-erbium fiber may replace the ytterbium-erbium co-doped phosphatic glass rod in order to provide a low lasing threshold that is compatible with pumping by a low-power Nd:YAG laser, which in turn is pumped by a diode. The low threshold characteristics of the fiber are thought by Hanna et al. to be one modification of their experimental laser that would increase pumping efficiency and allow a low-power Nd:YAG laser to be used as a pump source while maintaining reasonable efficiency characteristics for the co-doped erbium glass laser. Like all bulk laser glass, however, the single-pass gain of the co-doped erbium glass rod laser measured by Hanna et al. is low and, therefore, does not suggest the co-doped glass could be successfully employed in a fiber of an optical amplifier to provide high gain and/or high output saturation power.